The present invention relates to a method for producing a semiconductor device having elemental semiconductor units monolithically integrated in a semiconductor body, with the units having a common or a plurality of common zones of one conductivity type with locally reduced thickness and pn-junctions between zones of different conductivity types. More particularly, the present invention relates to a method of forming such a semiconductor device wherein the method comprises producing the zone structures of the elemental units by diffusing doping substances into the semiconductor body from one and/or both major surfaces of the same in time stages with the use of a masking technique, wherein the masking layer is etched away by an etching medium at least at one major surface of the semiconductor body above the region of the semiconductor body in which the thickness of one zone of the device is to be reduced.
In a so-called thyristor with integrated diode made according to such a known process, e.g. see German Offenlegungsschrift (Laid Open Application) No. 23 60 081, a zone of the semiconductor body which is common to the two semiconductor units, i.e., the thyristor and the rectifying diode, advisably has a lesser thickness in the region of the diode than in the region of the thyristor. The respective common zone is, for example, n-conductive, and forms the anode side n-conductive base zone in the region of the semiconductor body forming the thyristor and the cathode side n-conductive base zone in the region of the semiconductor body forming the diode. This common n-conductive zone is adjacent, in the region of the semiconductor body forming the thyristor, to the p-conductive anode zone and, in the region of the semiconductor body forming the diode, to an n.sup.+ -conductive cathode zone which has a greatly reduced thickness in the dimension which determines the n-conductive base zone.
The zone structure of the known semiconductor device may be produced, for example, in the following proces steps. A wafer-shaped silicon semiconductor body of n-type conductivity is provided with an oxide layer on its two major surfaces and this oxide layer is covered by a photolacquer layer. Then the photolacquer layer is removed from one major surface above an annular edge region of the semiconductor wafer in which the thickness of the base zone of the diode is to be reduced, and thereafter the exposed portion of the oxide layer is etched away and the remainder of the photolacquer layer is removed. Into this semiconductor wafer, which has been thus prepared for the diffusion process and has been provided with an oxide masking layer, phosphorus is then diffused and the above-mentioned n.sup.+ cathode zone is preshaped in the region of the diode. Thereafter, gallium is diffused into the wafer from both major surfaces through the masking layer and thus a pnp zone structure is formed in the region of the semiconductor wafer constituting the thyristor and the finished n.sup.+ np diode structure with an n.sup.+ cathode zone having a greater thickness is formed in the region of the wafer constituting the diode. After etching away a, for example, circular mask opening in the previously closed oxide layer of the other major surface of the wafer, phosphorus is finally again diffused into the wafer to form the n.sup.+ cathode zone of the thyristor.
In this process the diffusion step to form a sufficiently thick n.sup.+ cathode zone for the diode, which step takes place at high temperature, takes a very long time, for example about 50 hours, which has certain drawbacks. For example, a faulty oxide masking layer can have particularly disadvantageous effects. Moreover, in this process the life time of the charge carriers is reduced to an undesirable degree.